1. Field of the Invention
The present invention relates to a control apparatus for controlling the speed ratio in a belt-type continuously variable transmission. More particularly, it relates to a control apparatus for electrically controlling the speed ratio.
2. Relating Prior Arts
Belt-type continuously variable transmissions are previously well-known and have been put to practical use. It comprises, for example, drive and driven pulleys each having a variable pulley width and a metallic V-belt trained around the drive and driven pulleys.
In the belt-type continuously variable transmission, the speed ratio (or gear ratio) is controlled by controlling the pulley widths of the drive and driven pulleys. The drive and driven pulleys respectively have drive and driven hydraulic cylinders for applying thrust forces to the pulleys, which forces are produced by a hydraulic pressure supplied to the cylinders. The control of the thrust forces to the respective cylinders allows varying the diameters of circles around which the V-belt is trained on the pulleys to control the speed ratio of the transmission. The control of the thrust forces also allows a tension control of the V-belt. One example of the speed ratio control apparatus for a belt-type continuously variable transmission is disclosed in Japanese Laid-open Patent Publication No. S61-206862.
Such a belt-type continuously variable transmission as described above can be mounted on an automobile. In the transmission mounted on an automobile, the hydraulic pressure supplied to the hydraulic cylinder to produce the thrust force is often controlled electrically. The hydraulic pressure is regulated by solenoid valves or solenoid actuators which are operationally controlled based on electric signals corresponding to an engine throttle opening and an engine speed. One example of such a control apparatus is disclosed in Japanese Laid-open Patent Publication No. S63-215437.
In a control apparatus in which the hydraulic pressure to produce the thrust force is electrically controlled, any electric trouble may deteriorate the controllability of speed ratio in the transmission. The control apparatus disclosed in Japanese Laid-open Patent Publication No. H4-243634 has means to enable a supplemental speed ratio control under an electric failure condition. The control apparatus has at least two parallel hydraulic control passages to supply a pulley width control pressure to the drive and driven cylinders. One of the control passage is used under normal conditions and the other control passage is used when any electric failure occurs.
In the control apparatus, control valves are needed in parallel in both of the control passages and the construction thereof tends to become complicated. Further, since many valves are disposed in the control passage through which a relatively high hydraulic pressure is supplied, the amount of leakage may easily increase. When the leakage is increased, a bigger hydraulic pump is needed to supply more oil. Since the bigger pump needs more drive power, the fuel economy of the engine may be deteriorated.